This invention relates to a vandal proof system for installation in public/low cost housing where existing intercom units are typically telephone handsets.
In such installations, landlords find intercoms costly, lacking durability, and require routine maintenance. One of the more significant problems encountered with telephone apartment stations of the foregoing type has arisen from vandalism. With increasing frequency, the handsets have been ripped off, the cords cut, and the dials or buttons rendered inoperative such as by inserting various instruments or a strong adhesive between the moving parts of the apparatus. In some cases, the cover has been pried loose and the interior components damaged or removed. These problems have necessitated frequent monitoring and maintenance of the individual stations which landlords are not able to address.
The following references relating to intercom systems are known in the art: U.S. Pat. No. 353,282 to Holm; U.S. Pat. No. 3,665,116 to Holstrom; U.S. Pat. No. 4,196,317 to Bartelink U.S. Pat. No. 4,764,953 to Chern; and U.S. Pat. No. 5,966,432 to Buckler.
In the past, some apartment switching units were installed where apartment intercoms were furnished and maintained by the Bell telephone company. Thus, many elements of apartment station intercom units and apartment switching units functionally correlate to telephone sets and a telephone network. Since the break-up of Bell Telephone in 1983 into regional operating companies, (RBOCs), responsibility for intercom maintenance has gradually fallen to landlords. Now, landlords would prefer to replace these telephone units with stand alone intercoms requiring minimal or no maintenance, as well as being durable enough to withstand tenant abuses and vandalism.
The interface between the system apartment station intercom unit and the apartment building central intercom switch is similar to a POTS system, wherein POTS is an acronym for a “plain old telephone” system. This POTS system is essentially a twisted pair with a 48 Volt open loop potential. In a phone system, when the receiver is lifted (off hook), the local telephone set is engaged or turned on, thus drawing power from the twisted pair. This off hook condition signals the telephone central office, which then, in turn supplies a dial tone. This process and system is called a loop start system, and has been the basis of telephony for nearly a century. Since the apartment station intercom unit interfaces with a similar system, it is required to perform many of the same functions as a telephone.
The apartment station intercom unit has a ring signal detector and an electronic ringer. This ring signal detector alerts a tenant that someone in the entrance has dialed his or her apartment. To establish communication with the call, the tenant presses the Push-To-Talk(PTT) button on the Apartment Station Intercom Unit. This action does three things, first it signals the apartment building central switch that the system or an apartment station intercom unit is “off hook”. In addition, it also enables the transmit circuit and starts a timer.
When the tenant releases the PTT switch, he or she can hear the caller. Once the tenant decides that the caller may enter the building, the door release button is pressed, signaling the apartment building central switch to release the door lock. This is a the Bell Standard Dual Tone Multi-Frequency(DTMF) signal.
The timer that was initiated when the tenant first pressed the PTT button, and prevents the tenant from engaging the caller in a prolonged conversation, which is considered an abuse of the system.